1. Field of the Invention
The invention relates to a PWM signal generating circuit that is able to generate a high-resolution PWM signal without using a high-speed clock.
2. Description of the Related Art
A PWM wave generating circuit that controls an output voltage is described in, for example, Japanese Patent Application Publication No. JP-A-2001-169541. The PWM wave generating circuit described in this publication includes a constant current circuit in which a current value changes based on a feedback voltage; a capacitor that is charged with electric currents from the constant current circuit; a switch circuit that releases an electric charge of the capacitor; an A/D (analog-to-digital) converter that receives the feedback voltage and transmits a digital signal to a delay circuit; the delay circuit that receives the digital signal and controls the switch circuit; and a first comparator that controls a voltage of a PWM wave using the voltage in the capacitor.
Also, Japanese Patent Application Publication No. JP-A-2004-345280 describes a PWM signal generating circuit that has ON/OFF information used for generating a PWM signal that is able to be resolved into units for a reference clock, a counter that performs counting using the reference clock, and a selector circuit or a comparator circuit that selects the ON/OFF information corresponding to each count value that the counter may take and outputs the selected ON/OFF information. This PWM signal generating circuit is formed of a waveform synthetic circuit that forms a PWM signal by synthesizing a signal output from the selector circuit or the comparator circuit. The waveform synthetic circuit latches, using a base clock, the results obtained by computing the results of comparisons made by the comparator circuit or the results of selections made by the selector circuit within a time range that is equal to two or more units; further latches, using the base clock, the results each of which is obtained by computing at least two different results of latching; repeats the same operation until the latching becomes one; and uses the output of the one latching as the PWM signal of the basic clock resolution.
As with the configuration described in Japanese Patent Application Publication No. JP-A-2004-345280, in a common digital PWM signal generating circuit, a high-speed clock needs to be used in order to generate a high-resolution PWM signal. This increases the cost of the circuit configuration.
Meanwhile, in an analog PWM signal generating circuit, a leading and a triangular wave (e.g. a ramp wave), which are used to set a duty ratio of a PWM signal, are compared with each other. To generate a high-resolution PWM signal, a highly accurate reading and a good triangle wave need to be realized. Also, great attention needs to be given so that the reading and the triangle wave are not influenced by noise, etc. In addition, a comparator with low drift and low offset is usually expensive. This increases the cost of the circuit configuration, as with the digital PWM signal generating circuit.
With the art described above, the resolution of the PWM signal is attempted to be increased (discrete pulse duration is made continuous by a digital signal) by using an A/D converter that transmits a digital signal to the delay circuit. However, the resolution of the PWM signal is not uniformly increased with the art described above.